Generally speaking feathers include impurities such as dust attached thereto. In order to separate the impurities, the feathers, after being subjected to a washing process to remove oil therefrom and to a drying process, are supplied to a mixing box according to the prior art. The feathers are then mixed with each other in the mixing box to cause the impurities to drop toward the bottom of the box. However in such a prior art, the feathers are easily brought together with each other and the webs of each of the feathers are apt to be in contact with each other. Therefore, the prior art has a disadvantage in that the impurities attached to the feathers cannot be easily separated therefrom, and that the resultant feathers lack bulkiness. In addition to this, since the impurities are attached to the feathers by an electro-static force, such impurities cannot be effectively separated by the mechanical mixing procedure. Furthermore, since the eggs of bird lice or ticks attached firmly to the feathers cannot be removed therefrom or killed by the prior art, the resultant feathers exhibit an inferior and undesirable quality. Still further, since the cleaning process in the prior art is carried out batchwise, the feathers from the preceding processes such as washing and drying are not continuously introduced into the cleaning process, which causes a low output efficiency in the treatment of the feathers.